<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style>
        .top{
            width: 500px;
            height: 500px;
            background-color: #18bebe;
            position: relative;
        }
        .left{
            width: 200px;
            height: 200px;
            background-color: #ff8201;
            position: absolute;
            top: 50%;left: 50%;
            margin-left: -100px;margin-top: -100px;
        }
    </style>
</head>
<body>
<div class="top">
    <div class="left"></div>
</div>

</body>
</html>